Uptake of Plasma Amino-acids by the Perfused Isolated Cow's Udder

Abstract

ARTERIO-VENOUS studies carried out on lactating udders of living cows have repeatedly led to the conclusion that the free amino-acids of the circulating blood are taken up by the gland₁. Most authors agree that the observed arterio-venous differences in free amino-acids are inadequate to account quantitatively for the synthesis of the milk proteins—casein and lactalbumin. Other workers found an uptake of protein-bound carbohydrates by the udder of the lactating goat². No uptake of amino-acids by the udder of the goat and the cow was found during inanition even though some milk was still being secreted². Recently, carbon-14 labelled valine and carbon-14 labelled lysine were injected intravenously into lactating rabbits³, and the activity of milk and blood proteins estimated. In both cases it was found that the milk proteins were highly radioactive. As this activity was much greater than that of the blood proteins, it was clear that the free amino-acids of the blood were being incorporated directly into the milk proteins.